With a widespread use of a so-called multimedia technique in recent years, a huge volume of data including digital still images, motion pictures, etc. have been handled. Such a huge volume of data is stored in a recording medium of a large capacity, such as an optical disk, and reproduced as necessary by means of random access. The optical disk has a merit that it can provide random access and a higher recording density than that of a magnetic recording medium, such as a floppy disk.
Further, some kinds of optical disks, such as a magneto-optical disk, are erasable (overwritable) and used extensively as recording media in handling a huge volume of data including digital still images and motion pictures, etc.
Most of the magneto-optical disks are provided with pits and projections on its recording layer, which are referred to as grooves and lands and used as tracking guides.
The pits and projections are also used to create address information of the track beforehand. In other words, a carrier of a specific frequency is modulated by a cluster number or a sector number indicating a position on the recording medium, and the projection forming the grooves is wobbled in accordance with this modulation signal in advance, so that the shape (wobbled shape) of the side wall of the grooves thus formed indicates address information of the track.
In order to record an increased volume of data into the recording medium, such as an optical disk and a magneto-optical disk, data is recorded at a high density by, for example, giving a narrower track pitch and thereby increasing a linear density in the track direction.
The following will explain an example optical disk disclosed in Japanese Laid-open Patent Application No. 259441/1997 (Japanese Official Gazette, Tokukaihei No. 9-259441, publishing date: Oct. 3, 1997) (hereinafter, referred to as Document 1) with reference to FIGS. 11 and 12.
As shown in FIG. 11, an optical disk 110 is provided with a wobbling groove G1 indicated by a broken line with both side walls being wobbled and thereby creating address information beforehand and a non-wobbled DC groove G2 indicated by a solid line, which are independent continuous spirals heading from the inner radius to the outer radius of the optical disk 110.
In other words, the optical disk 110 is provided with the wobbling groove G1 and DC groove G2 aligned alternately along the radius direction, and as shown in FIG. 12, information is recorded into lands L1 and L2 formed between the wobbling groove G1 and DC groove G2.
The wobbling groove G1 and DC groove G2 are aligned alternately per rotation, which means two-track pitch is secured between the adjacent wobbling grooves G1. This arrangement can suppress crosstalk (interference from the side wall of the other tracks) caused when reading the address information from the side wall of the wobbling groove G1.
In addition, because the wobbling groove G1 or DC groove G2 is provided between the adjacent lands L1 and L2, crosserase can be also suppressed. In this manner, a recording medium having a narrower track pitch and hence a higher recording density can be realized.
Incidentally, the address information of a particular track, for example, the address information of the lands L1 and L2 of FIG. 12 is formed as the shape of the wobbling groove G1 provided at the inner or outer radius of the lands L1 and L2. In other words, the land L1 at the inner radius of the wobbling groove G1 and the land L2 at the outer radius of the wobbling groove G1 share the address information.
Hence, when information is recorded/reproduced into/from the optical disk 110, it is necessary to conduct track area judgment (wobbling polarity judgment), by which whether the address information of an area being tracked is the one for a first track area (land L1) at the outer radius of the wobbling groove G1 or a second track area (land L2) at the inner radius of the wobbling groove G1 is judged.
In case that three laser beams (a main beam used in recording/reproducing data and two sub-beams used in detecting a tracking error) are used, the track area judgment is generally conducted by using reflection light of the two sub-beams. For example, as shown in FIG. 12, a main beam MB1 is irradiated at the center of the first track area (land L1), while a first sub-beam SB1 is irradiated at the center of the wobbling groove G1 at the inner radius of the land L1 and a second sub-beam SB2 is irradiated at the center of the DC groove G2, and a tracking error signal is detected by means of the DPP (Differential Push Pull) technique. Then, the track area judgment (wobbling polarity judgment) is conducted by comparing a wobble signal obtained from reflection light of the first sub-beam SB1 with a wobble signal obtained from reflection light of the second sub-beam SB2.
Here, as shown in FIG. 12, in case that the first sub-beam SB1 preceding the main beam MB1 is irradiated at the inner radius and the second sub-beam SB2 following the main beam MB1 is irradiated at the outer radius, if the wobble signal obtained from the first sub-beam SB1 is greater than the one obtained from the second sub-beam SB2, then it is judged that the wobbling groove G1 is at the inner radius of the disk in comparison with the main beam MB1, and therefore, the area being tracked is the first track area (land L1) at the outer radius of the wobbling groove G1.
However, this technique demands three laser beams, and has a problem that it can not be realized by using an optical pick-up emitting only one laser beam.
In addition, because the wobble signal obtained from the first sub-beam SB1 is compared with the one obtained from the second sub-beam SB2 in largeness, the positional relation among the first sub-beam SB1 and second sub-beam SB2 and the track has to be set precisely.
Further, when using three laser beams, the irradiation intensity of the first sub-beam SB1 and second sub-beam SB2 has to be set to approximately 10% of the irradiation intensity of the main beam MB1, so that the first sub-beam SB1 and second sub-beam SB2 will not erase recorded data when recording a signal. This lowers an S/N ratio of output signals obtained from the first sub-beam SB1 and second sub-beam SB2, thereby causing a problem that an error readily occurs in the track area judgment.
In order to solve this problem, Document 1 discloses a technique, by which the track area judgment is conducted not by using three laser beams but only one laser beam. For example, according to this publication, one laser beam is irradiated at the position to which the main beam MB1 is irradiated in FIG. 12, and, in the detecting optical system, reflection light of the main beam MB1 is detected by a photo-detector divided into two sections by a dividing line along the track direction. Then, the track area judgment is conducted by comparing a wobble signal detected from the inner half-round area with a wobble signal detected from the outer half-round area.
Also, Japanese Laid-open Patent Application No. 40549/1998 (Japanese Official Gazette, Tokukaihei No. 10-40549, publishing date: Feb. 13, 1998) (hereinafter, referred to as Document 2) discloses a technique using only one laser beam, by which the track area judgment is conducted by comparing a wobble signal obtained when detracking a track toward the inner radius and a wobble signal obtained when detracking a track toward the outer radius.
However, the technique disclosed in Document 1 has a problem that a correspondence of the light receiving sections to a detection area inverts depending on whether the photo-detector is placed before or after a focal point of the detecting optical system.
In addition, because each apparatus is different from the others, a difference should be corrected by replacing wiring or changing switch setting, thereby causing a problematic increase in the manufacturing costs.
Further, given λ as a wavelength of a light source used for an optical head of the recording and reproducing apparatus, then the large-and-small relation of the wobble signals inverts depending on whether the depth of the groove of the recording medium is greater or smaller than λ/4. This problematically limits the depth of the groove of a used recording medium.
Also, according to the technique disclosed in Document 2, the track area judgment is conducted after the wobble signals are measured at least in two tracking states, which makes a real-time judgment impossible. Thus, in case that the first and second track areas are wobbled by the common address information, even if an unwanted track jump occurs, it can not be detected from the address information. Therefore, there rises a problem that when a signal is recorded, recorded data is broken, and when a signal is reproduced, data can not be reproduced continuously.